Method of producing tube of duplex stainless steel

ABSTRACT

Method of producing a tube of duplex stainless steel is disclosed. The steel comprises the following composition, in weight %: C max 0.03, Si max 1.0, Mn max 1.5, P max 0.05, S max 0.03, Cr 24-26, Ni 6-8, Mo 3.0-4.0, N 0.24-0.32. The method comprises steps of: forming a tube of the duplex stainless steel, cold working the tube obtained from the step of forming a tube, and soft annealing the tube after the step of cold working by subjecting the tube to a temperature, T, within a range of 500-750° C. for a time period, t, of 0.5-5 minutes.

TECHNICAL FIELD

The present disclosure relates to a method of producing a tube of duplex stainless steel.

BACKGROUND

During the production of a tube of duplex stainless steel, the tube is subjected to cold working to reach the desired geometry and tolerances. The cold working may also be used to deformation harden the tube to a specified minimum yield strength. Since duplex steel deformation hardens rapidly along with area reduction during cold working, it is difficult to achieve both a specified geometry and the specified minimum yield strength in one cold working operation. Thus, in order to provide a duplex stainless steel tube with specific geometry and minimum yield strength the duplex stainless steel tube is cold worked and thereafter annealed at approximately 1100° C. causing a full recrystallisation of the duplex stainless steel. Thereafter, the duplex stainless steel tube is subjected to a second cold working step with a limited area reduction. The limited area reduction is chosen such that the specific minimum yield strength is achieved.

EP 2853614 discloses a duplex stainless steel pipe and a manufacturing method thereof. Cold working of the duplex material introduces anisotropy in the pipe. A low temperature heat treatment is performed at a heat treatment temperature of 350° C. to 450° C. Thus, the anisotropy of the yield strength in the pipe axis direction and in the pipe circumferential direction of the stainless steel pipe is decreased after the cold working.

WO 2017/114847 discloses a process of producing a duplex stainless steel tube. The process is based around formulas taking into account the constituents of the duplex stainless steel as well as parameters of the area reduction during cold working in order to provide a target yield strength.

SUMMARY

It is an object of the present disclosure to provide an efficient method of producing a tube of stainless steel.

According to an aspect of the present disclosure, there is provided a method of producing a tube of duplex stainless steel comprising the following composition, in weight %:

C max 0.03,

Si max 1.0,

Mn max 1.5,

P max 0.05,

S max 0.03,

Cr 24-26,

Ni 6-8,

Mo 3.0-4.0,

N 0.24-0.32, and

a balance of Fe and unavoidable impurities, wherein

the method comprises steps of:

-   -   forming a tube of the duplex stainless steel,     -   cold working the tube obtained from the step of forming a tube,         and     -   soft annealing the tube after the step of cold working by         subjecting the tube to a temperature, T, within a range of         500-750° C. for a time period, t, of 0.5-5 minutes.

Since the tube of stainless steel which has been cold worked, is soft annealed by subjecting the tube to a temperature within a range of 500-750° C. for a time period of 0.5-5 minutes, recrystallisation of the cold worked tube is substantially avoided. The yield strength of a duplex stainless steel tube produced in accordance with this method is improved over that of the duplex stainless steel material before the step of cold working while ductility remains at an adequate level, without becoming too brittle. As a result, the above mentioned object is achieved.

Moreover, a particular minimum yield strength may be targeted without having to anneal the duplex stainless steel tube to full recrystallisation and without having to subject the duplex stainless steel tube to a second cold working step. Thus, the present disclosure is based around one single step of cold working providing a high area reduction followed by soft annealing.

When subjected to temperatures within a range of 450-1000° C., duplex stainless steel materials are prone to embrittlement and reduced corrosion resistance. Therefore, commonly this temperature range has been avoided during the manufacturing and use of duplex stainless steel tubes.

The inventors have realised that subjecting a duplex stainless steel tube to the temperature range of 500-750° C. for a short controlled time period of 0.5-5 minutes will improve the ductility of the duplex stainless steel tube without causing embrittlement or reduced corrosion resistance. Moreover, the inventors have discovered that a target minimum yield strength may be achieved in a controlled manner.

The target minimum yield strength may be achieved by the step of soft annealing at the above mentioned temperature range and within the above mentioned time period. In contrast to one of the above described prior art methods, this is achieved without requiring annealing at a high temperature causing full recrystallisation followed by a second cold working step. In the present disclosure the step of soft annealing results in a partial reduction of the yield strength achieved in the step of cold working, to arrive at a target minimum yield strength, see further below.

The above specified duplex stainless steel material also goes under the name, Sandvik SAF2507®. One common use of such duplex stainless steel tubes is within the oil and gas industry.

The step of forming the tube of the duplex stainless steel results in a tube ready for the cold working step. The step of forming the tube may comprise the producing of an ingot or a billet from a melt. The ingot or billet is then hot extruded into a tube. Alternatively, the step of forming the tube may comprise the production of a duplex stainless steel powder, which is formed and hot isostatic pressed into a tube shape.

The step of cold working the tube may be performed in a known manner such as by cold rolling, cold pilgering, cold drawing, or combinations thereof. The step of cold working may be the only cold working step in the method of producing a tube of duplex stainless steel. As such, the step of cold working may comprise cold working to a final dimension of the tube, or to substantially the final dimension of the tube. Only a minor diameter adjustment may take place subsequent to the step of cold working, as a result of e.g. peel turning, grinding, and/or polishing.

According to embodiments, in the step of soft annealing the tube, a target minimum yield strength, Rp0.2, of the duplex stainless steel may be achieved following the formula Rp0.2=1642−0.96×T−10.75×t with a standard deviation in the target minimum yield strength, Rp0.2, of 10 MPa, wherein T is temperature in degrees Celsius, t is time in minutes. In this manner, a particular minimum yield strength may be achieved in the step of soft annealing. The unit of Rp0.2 is MPa.

More specifically, by selecting an appropriate temperature and an appropriate time period within the above discussed temperature and time period ranges, a target minimum yield strength Rp0.2 within a range of 868.25-1156.25 MPa may be achieved with a standard deviation of 10 MPa for a duplex stainless steel tube of the above defined material, Sandvik SAF2507®. Thus, a particular minimum yield strength may be easily targeted without having to anneal the duplex stainless steel tube to full recrystallisation and without having to subject the duplex stainless steel tube to a second cold working step.

According to embodiments, the step of soft annealing the tube may be followed by a step of:

-   -   quenching the tube. In this manner, it may be ensured that the         step of soft annealing is interrupted at a particular point in         time.

Further features of, and advantages will become apparent when studying the appended claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and/or embodiments, including particular features and advantages, will be readily understood from the example embodiments discussed in the following detailed description and the accompanying drawings, in which:

FIG. 1 illustrates a tube of duplex stainless steel,

FIG. 2 illustrates a method of producing a tube of duplex stainless steel.

FIG. 3 illustrates a diagram over the yield strength of test specimens of a tube of duplex stainless steel produced in accordance with the method.

DETAILED DESCRIPTION

Aspects and/or embodiments will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity.

FIG. 1 illustrates a duplex stainless steel tube 2 according to embodiments. The duplex stainless steel tube 2 has an outer diameter D, and a wall thickness w. The duplex stainless steel tube 2 has been produced by a method of producing a tube of duplex stainless steel according to aspects and/or embodiments discussed herein.

Mentioned purely as an example, the outer diameter D of the tube 2 may be within a range of 50-250 mm. A wall thickness w of the tube 2 may be within a range of 5-35 mm.

FIG. 2 illustrates a method 10 of producing a tube of duplex stainless steel. The tube may be a tube 2 as discussed in connection with FIG. 1.

The duplex stainless steel comprises the following composition, in weight %:

C max 0.03,

Si max 1.0,

Mn max 1.5,

P max 0.05,

S max 0.03,

Cr 24-26,

Ni 6-8,

Mo 3.0-4.0,

N 0.24-0.32, and

a balance of Fe and unavoidable impurities.

As to the composition of the duplex stainless steel, the following is to be noted regarding the individual alloying elements therein:

Carbon, C is a representative element for stabilizing austenitic phase and an important element for maintaining mechanical strength. However, if a large content of carbon is used, carbon will precipitate as carbides and thus reduces corrosion resistance. According to one embodiment, the carbon content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is 0 to 0.03 wt % such as 0 to 0.02 wt %.

Chromium, Cr, has strong impact on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter, especially pitting corrosion. Cr improves the yield strength, and counteracts transformation of austenitic structure to martensitic structure upon deformation of the duplex stainless steel. However, an increasing content of Cr will result in for the formation of unwanted stable chromium nitride and sigma phase and a more rapid generation of sigma phase. According to one embodiment, the chromium content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 24 to 26 wt %.

Manganese, Mn, has a deformation hardening effect on the duplex stainless steel as defined hereinabove or hereinafter. Mn is also known to form manganese sulphide together with sulphur present in the steel, thereby improving the hot workability. However, at too high levels, Mn tends to adversely affect both corrosion resistance and hot workability. According to one embodiment, the manganese content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is 0 to 1.5 wt % such as 0 to 1.2 wt %.

Molybdenum, Mo, has a strong influence on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and it heavily influences the pitting resistance equivalent, PRE. Mo has also a positive effect on the yield strength and increases the temperature at which the unwanted sigma-phases are stable and further promotes generation rate thereof. Additionally, Mo has a ferrite-stabilizing effect. According to one embodiment, the molybdenum content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 3.0 to 4.5 wt % such as 3.5 to 4.0 wt %.

Nickel, Ni, has a positive effect on the resistance against general corrosion. Ni also has a strong austenite-stabilizing effect. According to one embodiment, the nickel content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 6 to 8 wt %, such as 6.5 to 7.5 wt %.

Nitrogen, N, has a positive effect on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and also contributes to deformation hardening. It has a strong effect on the pitting corrosion resistance equivalent PRE (PRE=Cr+3.3Mo+16N) and has also a strong austenite stabilizing effect and counteracts transformation from austenitic structure to martensitic structure upon plastic deformation of the duplex stainless steel. However, at too high levels, N tends to promote chromium nitrides, which should be avoided due to their negative effect on ductility and corrosion resistance. According to one embodiment, the nitrogen content of the duplex stainless steel used in the method disclosed hereinabove or hereinafter is 0.24 to 0.32 wt % such as 0.26 to 0.30 wt %.

Silicon, Si, is often present in the duplex stainless steel since it may have been added for deoxidization earlier in the production thereof. Too high levels of Si may result in the precipitation of intermetallic compounds in connection to later heat treatments or welding of the duplex stainless steel. Such precipitations will have a negative effect on both the corrosion resistance and the workability. According to one embodiment, the silicon content of the duplex stainless steel used in the method disclosed hereinabove or hereinafter is of 0 to 1.0 wt % such as 0 to 0.8 wt %.

Phosphorous, P, may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter, and will result in deteriorated workability of the steel if at too high level, thus, P<0.05 wt % such as P<0.03 wt %.

Sulphur, S, may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter and will result in deteriorated workability of the steel if at too high level, thus, S<0.03 wt % such as S<0.02 wt %.

Oxygen, O, may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter, wherein O<0.010 wt %.

Optionally small amounts of other alloying elements may be added to the duplex stainless steel as defined hereinabove or hereinafter in order to improve e.g. the machinability or the hot working properties, such as the hot ductility. Example, but not limiting, of such elements are REM, Ca, Co, Cu, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce. The amounts of one or more of these elements are of max 0.5 wt %. According to one embodiment, the duplex stainless steel as defined hereinabove or hereinafter may also comprise small amounts other alloying elements which may have been added during the method, e.g. Ca (<0.01 wt %), Mg (<0.01 wt %), Cu (max 0.5 wt %), and rare earth metals REM (<0.2 wt %).

When the term “max” is used, the skilled person knows that the lower limit of the range is 0 weight % unless another number is specifically stated. Examples of impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the raw material or the additional alloying elements used for manufacturing of the duplex stainless steel.

The method 10 comprises steps of:

-   -   Forming 12 a tube of the duplex stainless steel.     -   Cold working 14 the tube obtained from the step of forming 12 a         tube.     -   Soft annealing 16 the tube after the step of cold working 14 by         subjecting the tube to a temperature, T, within a range of         500-750° C. for a time period, t, of 0.5-5 minutes.

According to some embodiments, the step of forming 12 a tube may comprise steps of:

-   -   producing 20 an ingot or a continuous casted billet of the         duplex stainless steel, and     -   hot extruding 22 the ingot or the billet obtained from the step         of producing 20 an ingot or a continuous casted billet into a         tube.

In this manner, a tube of the duplex stainless steel may be provided for the subsequent step of cold working 14 the tube.

The method 10 may include further known intermediate and subsequent production steps such as cutting, peel turning, deep boring, straightening, and acid pickling.

According to alternative embodiments, the step of forming 12 a tube may comprise steps of:

-   -   providing 24 a melt of the duplex stainless steel,     -   atomizing 26 the melt of the duplex stainless steel to produce a         powder of the duplex stainless steel,     -   forming 28 the tube of the powder of the duplex stainless steel,         wherein the step of forming 28 the tube of the powder comprises         a step of:     -   hot isostatic pressing 30 the powder of duplex stainless steel.

In this manner, a tube based on a powder duplex stainless steel material may be produced and provided for the subsequent step of cold working 14 the tube.

The method 10 may include further known intermediate and subsequent production steps such as cutting, peel turning, straightening, and acid pickling.

The step of soft annealing 16 the tube may be performed e.g. in a furnace under controlled temperature conditions. A further alternative may be to induction heat the tube to a specified temperature.

The step of soft annealing 16 the tube may be performed in accordance with a particular formula in order to achieve a particular target minimum yield strength, Rp0.2, in the tube of the duplex stainless steel. The formula reads:

Rp0.2=1642−0.96×T−10.75×t

In the formula T represents the temperature in degrees Celsius, t represents the time period in minutes, which results in that the targeted minimum yield strength, Rp0.2, has the unit MPa. A standard deviation of 10 MPa is achieved in the target minimum yield strength, Rp0.2. As mentioned above, for the material Sandvik SAF2507® a target minimum yield strength Rp0.2 within a range of 868.25-1156.25 MPa may be achieved with a standard deviation of 10 MPa when the step of soft annealing 16 the tube is performed in accordance with the formula.

Suitably, the step of soft annealing 16 the tube may be followed by a step of:

-   -   quenching 18 the tube. Thus, the step of soft annealing may be         ended, and it may be ensured that embrittlement and reduced         corrosion resistance is avoided. The quenching 18 may be         performed by quenching in water.

According to some embodiments, the step of soft annealing 16 the tube may comprise subjecting the tube to a temperature within a range of 600-750° C. for a time period of 1-3 minutes.

Below, with reference to FIG. 3 examples related to these embodiments are discussed.

According to embodiments, the step of cold working 14 the tube, includes an area reduction of the tube within a range of 25-70%. In this manner, the duplex stainless steel may be strain hardened in the step of cold working 14 the tube.

This is a range of area reduction comparable to that of the first cold working step performed in the prior art method discussed under the heading Background above, but considerably larger than the second cold working step of the prior art method.

The area reduction relates to a reduction of the cross sectional area of the tube. The difference in cross sectional area before and after the step of cold working is divided by the cross sectional area before the step of cold working. Only the area of the tube where there is material is taken account of in the calculation of the area reduction, i.e. the cross sectional area of the inside of the tube is excluded.

With reference to FIG. 3 a number of examples related to duplex stainless steel tubes produced in accordance with the present method will now be discussed. In particular soft annealing in accordance with various embodiments of the step of soft annealing 16 of the method of 10 will be presented.

A duplex stainless steel tube of the above defined material, Sandvik SAF2507®, having an outer diameter, D of 173.8 mm and a wall thickness of 29.7 mm has been produced by the steps of forming 12 the tube, including producing an ingot or casted billet, and cold working 14 the tube. The step of cold working 14 was performed by cold pilgering with an area reduction of 30.7%.

The step of soft annealing 16 was performed by subjecting different lengthwise sections of the tube to different temperatures over varying time periods, followed by water quenching. More specifically, the different sections of the tube were subjected to induction heating using effect ranges 40-100 kW at a frequency of 30 kHz for different time periods utilizing different feeding rates through the induction heating apparatus.

The different sections of the tube were cut up into pieces, of which test specimens for tensile testing were manufactured. In FIG. 3 the yield strength in MPa of the differently numbered test specimens is shown.

The test specimens which have been subjected to the same soft annealing temperature and time have a yield strength within a limited yield range, as clearly visible in the diagram of FIG. 3. The eighth test samples within each circle have been subjected to the same temperature and time period.

Thus, the conclusion can be drawn that a consistent reduction in yield strength is achievable for each particular soft annealing temperature and particular soft annealing time period. Moreover, the above discussed formula for the reduction of the minimum yield strength of a tube of the duplex stainless steel Sandvik SAF2507® is verified by the yield strengths of the test specimens.

More specifically, the different encircled test specimens in FIG. 3 have been subjected to the following soft annealing temperatures and soft annealing time periods:

40-750° C. for 3 minutes

42-750° C. for 1 minutes

44-700° C. for 3 minutes

46-700° C. for 1 minutes

48-650° C. for 3 minutes

50-650° C. for 1 minutes

52-600° C. for 1 minutes

54—Test samples which have not been soft annealed after cold working.

The microstructure of the soft annealed tube had not recrystallised and was similar to, or almost similar to, that of the cold worked tube. Concerning the hardness of the soft annealed tube, it could be concluded that the hardness is on same level as low reduction cold worked Sandvik SAF2507® material, i.e. a hardness comparable to that of a tube made of Sandvik SAF2507®, which has been produced according to a prior art method including a second step of cold working with an area reduction of less than 25%.

It is to be understood that the foregoing is illustrative of various example embodiments and that the scope of protection is defined only by the appended claims. A person skilled in the art will realize that the example embodiments may be modified, and that different features of the example embodiments may be combined to create embodiments other than those described herein, without departing from the scope of protection, as defined by the appended claims. 

1. A method of producing a tube of duplex stainless steel comprising the following composition, in weight %: C max 0.03, Si max 1.0, Mn max 1.5, P max 0.05, S max 0.03, Cr   24-26, Ni   6-8, Mo  3.0-4.0, N 0.24-0.32,

and a balance of Fe and unavoidable impurities, wherein the method comprises steps of: forming a tube of the duplex stainless steel, cold working the tube obtained from the step of forming a tube, and soft annealing the tube after the step of cold working by subjecting the tube to a temperature, T, within a range of 500-750° C. for a time period, t, of 0.5-5 minutes, and wherein, in the step of soft annealing the tube, a target minimum yield strength, Rp0.2, of the duplex stainless steel is achieved following the formula: Rp0.2=1642−0.96×T−10.75×t with a standard deviation in the target minimum yield strength Rp0.2 of 10 MPa, wherein T is temperature in degree Celsius, t is time in minutes.
 2. (canceled)
 3. The method according to claim 1, wherein the step of soft annealing the tube comprises subjecting the tube to a temperature within a range of 600-750° C. for a time period of 1-3 minutes.
 4. The method according to claim 1, wherein the step of soft annealing the tube is followed by a step of: quenching the tube.
 5. The method according to claim 1, wherein the step of cold working the tube, includes an area reduction of the tube within a range of 25-70%.
 6. The method according to claim 1, wherein the step of forming a tube comprises steps of: producing an ingot or a continuous casted billet of the duplex stainless steel, and hot extruding the ingot or the billet obtained from the step of producing an ingot or a continuous casted billet into the tube.
 7. The method according to claim 1, wherein the step of forming a tube comprises steps of: providing a melt of the duplex stainless steel, atomizing the melt of the duplex stainless steel to produce a powder of the duplex stainless steel, forming the tube of the powder of the duplex stainless steel in a process that includes hot isostatic pressing the powder of duplex stainless steel.
 8. The method according to claim 1, wherein the target minimum yield strength, Rp0.2, of the duplex stainless steel is 868.25-1156.25 MPa with a standard deviation of 10 MPa.
 9. The method according to claim 1, wherein the amount of Mn in the composition is 0 to 1.2 wt %.
 10. The method according to claim 1, wherein the amount of Mo in the composition is 3.5 to 4.0 wt %.
 11. The method according to claim 1, wherein the amount of Ni in the composition is 6.5 to 7.5 wt %.
 12. The method according to claim 1, wherein the amount of N in the composition is 0.26 to 0.30 wt %.
 13. The method according to claim 1, wherein the amount of Si in the composition is 0 to 0.8 wt %.
 14. The method according to claim 1, wherein the composition further includes: one or more of rare earth metals, Ca, Co, Cu, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce in a total amount of max 0.5 wt %.
 15. The method according to claim 1, wherein the composition further includes: Ca<0.01 wt %, Mg<0.01 wt %, Cu max 0.5 wt %, and rare earth metals <0.2 wt %. 